fcopy(): Add function, and use it instead of its pattern

[alejandro-colomar]

Cc: @stoeckmann

---

Revisions:

v1b

• Rebase

$ git rd 
1:  b80a2811e = 1:  64af7e26e lib/fs/copy/: fcopy(): Add function to copy the contents of a file into another
2:  9ce666a3d = 2:  ff6f6933b lib/, src/: Use fcopy() instead of its pattern

v2

• Add mask for mode_t in vipw(8) surrounding code. [@stoeckmann ]

$ git rd 
1:  64af7e26e = 1:  64af7e26e lib/fs/copy/: fcopy(): Add function to copy the contents of a file into another
2:  ff6f6933b = 2:  ff6f6933b lib/, src/: Use fcopy() instead of its pattern
-:  --------- > 3:  3bdea8b75 src/vipw.c: create_backup_file(): Use mask for mode_t

v2b

• Fix copyright year.

$ git rd 
1:  64af7e26e ! 1:  81ba76135 lib/fs/copy/: fcopy(): Add function to copy the contents of a file into another
    @@ lib/Makefile.am: libshadow_la_SOURCES = \
     
      ## lib/fs/copy/fcopy.c (new) ##
     @@
    -+// SPDX-FileCopyrightText: 2025, Alejandro Colomar <alx@kernel.org>
    ++// SPDX-FileCopyrightText: 2026, Alejandro Colomar <alx@kernel.org>
     +// SPDX-License-Identifier: BSD-3-Clause
     +
     +
    @@ lib/fs/copy/fcopy.c (new)
     
      ## lib/fs/copy/fcopy.h (new) ##
     @@
    -+// SPDX-FileCopyrightText: 2025, Alejandro Colomar <alx@kernel.org>
    ++// SPDX-FileCopyrightText: 2026, Alejandro Colomar <alx@kernel.org>
     +// SPDX-License-Identifier: BSD-3-Clause
     +
     +
2:  ff6f6933b = 2:  b37bbc39a lib/, src/: Use fcopy() instead of its pattern
3:  3bdea8b75 = 3:  27d38068a src/vipw.c: create_backup_file(): Use mask for mode_t

v2c

• Rebase

$ git rd 
1:  81ba76135 = 1:  0decb1cf3 lib/fs/copy/: fcopy(): Add function to copy the contents of a file into another
2:  b37bbc39a = 2:  27a3472c6 lib/, src/: Use fcopy() instead of its pattern
3:  27d38068a = 3:  8f22b1d3b src/vipw.c: create_backup_file(): Use mask for mode_t

v2d

• Rebase

$ git rd 
1:  0decb1cf3de4 = 1:  aa35b48a13e4 lib/fs/copy/: fcopy(): Add function to copy the contents of a file into another
2:  27a3472c6bbd = 2:  b0d9421e493c lib/, src/: Use fcopy() instead of its pattern
3:  8f22b1d3bfab = 3:  4049e3953483 src/vipw.c: create_backup_file(): Use mask for mode_t